How can I filter the values ​​from a drop-down menu of a custom field?

Hello everybody.

I've created a custom field in the Person that shows a drop-down menu.

The table of the custom field has a column with the uidDepartment reported.

Example:

custom table:

column1                                 column2

customValue1                       uid_department1

customValue2                       uid_department2

Person in Department2 --> DropDownMenu --> customValue2 (only this and not customvalue1)

How can I do to show in the drop-down menu only the values ​​that have that of the person as a department?

thanks a lot for the help